1. Field of the Invention
The present invention relates to a method according to the preamble of claim 1 of producing oxide films.
According to such a method a thin film containing silicon dioxide is produced on a growth substrate by an ALD method by bonding a vaporisable silicon compound onto the growth substrate and converting the bonded silicon compound to silicon dioxide.
The invention also relates to a method of producing multicomponent oxides (i.e. mixed oxides or tertiary oxides).
2. Description of the Related Art
The continual decrease in the size of microelectronics components is leading into a situation in which SiO2 can no longer be used as the gate dielectric (gate oxide) of MOSFET (metal-oxide-semiconductor field-effect transistor) since for achieving required capacitances the SiO2 layer should be made so thin that the tunneling current increases disadvantageously high from the functional point of view of the component. To avoid the problem SiO2 has to be replaced by a dielectric material with higher dielectric constant. In that case a thicker layer of the dielectric material than SiO2 can exist. Similarly the capacitance of DRAM (Dynamic Random Access Memory) capacitors must remain nearly constant meanwhile their decrease expeditiously in size, thus the previously used SiO2 and Si3N4 have to be replaced with materials having higher dielectric constants than these.
Materials having sufficiently high dielectric constants are abundant, but the problem is that the considered dielectric should be stable on the silicon surface, should most preferably be amorphous and should endure nearly unchanged under high post-treatment temperatures. Especially in the gate dielectric application a state where electrically active defects are rare should be provided at the interface of silicon and the high permittivity metal oxide. In the memory application the structure of the capacitor dielectric must be very stable due to the applied high activation temperatures. Due to the above mentioned facts it is preferable to admix SiO2 to the metal oxide with a higher dielectric constant.
In its various forms Chemical Vapor Deposition (CVD) is the most frequently used method of producing silicon dioxide (see patent publications JP 9306906, U.S. Pat. No. 4,845,054, U.S. Pat. No. 4,981,724, U.S. Pat. No. 5,462,899, JP 20868486, JP 6158329, JP 80061810, U.S. Pat. No. 4,872,947, JP 7026383, U.S. Pat. No. 5,855,957 and U.S. Pat. No. 5,849,644). Mainly tetraethoxy silane (TEOS) has been used as the silicon source material, and oxygen, water, hydrogen peroxide or ozone have been used as the oxygen source material in the patent publications. In the conventional CVD the oxygen source material is always brought simultaneously with the silicon source material to the growth substrate.
The conventional CVD method is related to the difficulty of controlling the process, and neither a sufficiently good coverage with the thin layers nor a good conformality is always achieved by CVD.
The invention is based to the idea that thin films containing silicon dioxide are produced by the Atomic Layer Chemical Vapor Deposition (ALCVD) process, which is generally known also as Atomic Layer Epitaxy (ALE) or Atomic Layer Deposition (ALD).
ALD is a current method of growing thin films (U.S. Pat. No. 4,085,430). According to the method a thin film is grown by means of saturable surface reactions, which are well separated from each other. The saturation is provided by means of chemisorption. In other words, the reaction temperature is selected as that the gaseous source material is stable at the growth temperature and additionally, it does not condense or decompose on the surface but is capable to react selectively with the reactive sites of the surface, e.g. with the OH groups or oxygen bridges (M-O-M) present on the oxide surface. OH groups functioning as reactive sites a so-called ligand exchange reaction takes place in which a covalent bond is formed between the surface and the source material (chemisorption). When the oxygen bridges are concerned a dissociating reaction takes place in which reaction a covalent bond is also formed (chemisorption). The bond formed by chemisorption is very strong and the surface structure formed on the surface is stable which enables the saturation of the surface by one molecular layer. The ligand exchange reactions are carried out by leading the gaseous or vaporised source materials alternately into the reactor and by purging the reactor with an inert gas between the pulses of the source materials (T. Suntola, Thin Solid Films 215 (1992) 84; Niinisto et al. Materials Science and Engineering B 41 (1996) 23). Also even and uniform films can be grown by ALD even on large surface areas. Accordingly films can be grown on both even and heterogeneous surface as well as on a grooved surface. Controlling the thickness and the composition of the film by means of the number of reaction cycles is precise and simple.
Silicon dioxide has also been gown by the ALD process. Compounds Si(NCO)4 and N(C2H5)3 (K. YamagucHi et al., Appl. Surf. Sci. (1998) 130-132) have been used as source materials. Producing silicon dioxide by Molecular Layer ALE and UHV-ALE processes using SiCl4 and H2O as source materials is also known in the literature (Surface Review and Letters, Vol. 6, Nos 3 & 4 (1999) 435-448).
The disadvantages of these known solutions are long reaction times, for what reason the proposed processes cannot be realized on an industrial scale.
The objective of the present invention is to eliminate the disadvantages related to the prior art and to provide a novel method, which enables a controlled growth of SiO2 containing thin films with sufficiently short reaction times.